A tail cloud is a narrow, often horizontal cloud band that attaches to a supercell's wall cloud and extends toward the storm's precipitation area. It marks air being pulled into the updraft from the rain-cooled forward flank.
How it forms
The forward flank of a mature supercell is a cool, humid pool created by falling rain and evaporation. Warm, moist inflow moving toward the updraft skims the top of this pool and rides along its boundary. Where that inflow is cooled and moistened enough to reach saturation, it condenses into a low ribbon of cloud that visibly runs from the precipitation region back to the wall cloud.
The shape can be laminar and tube-like or ragged and turbulent, depending on how strong the inflow is and how much rain-cooled air is mixing in. The tail cloud's long axis roughly follows the streamline of inflow entering the storm.
How spotters read it
A well-defined tail cloud is one of the clearest visual cues that a lowered cloud base is part of the storm's true updraft. Scud fragments beneath a rain shaft can look like a wall cloud from a distance, but scud is not attached to anything and does not have an inflow tail connecting it to the precipitation region.
Watching a tail cloud over several minutes also shows the inflow's behavior. Cloud elements streaming along the tail into the wall cloud, especially with rising motion at the wall cloud end, suggest that the updraft is being fed steadily. A tail cloud that goes flat and detaches often shows that inflow is being cut off.
Where it misleads
The presence of a tail cloud says nothing about rotation. Non-tornadic supercells and even strong multicells can display textbook tail clouds while producing no tornado. Rotation must be judged from the wall cloud itself.
In high-precipitation storms the tail cloud can be hidden inside the rain, and in low-precipitation storms it may not form at all because there is too little rain-cooled air to condense the inflow. Absence of a tail cloud is therefore not evidence that a storm is weakening.